1. Field of the Invention
This invention relates to cable and, in particular, to optical fiber cable.
2. Art Background
Optical fiber, as used in optical communication systems, is generally incorporated into a multifiber cable. Numerous multifiber cable configurations have been developed and each has been specifically designed to satisfy the requirements imposed by a particular application. Recent requirements have led to the encapsulation of the fibers within the cable, i.e., the fibers are enclosed by an elastomeric material. The encapsulation of the fibers is particularly advantageous in uses where the cable contacts liquids such as water. For example, undersea cables that include a self-supporting, load bearing wire cage, e.g., a steel wire cage, surrounding coated fibers that are encapsulated in an elastomer and an outer sheath, e.g., a nylon sheath, employ this encapsulated design. (See U.S. Pat. No. 4,156,104 issued May 22, 1979 which is hereby incorporated by reference.) If encapsulation is not utilized and the cable is ruptured, e.g., by fishing trawlers, or during recovery operations, water could enter the cable along the coated fibers and follow these fibers to electronic equipment, e.g., repeaters, spaced periodically along the transmission line. Water entering electronic equipment results in serious damage necessitating costly repairs and replacement. Additionally, the introduction of water into the cable increases the possibility of excess transmission losses. Thus, encapsulation to prevent water encroachment is certainly desirable.
The desire to exclude the presence of water is also strong for terrestrial applications. For example, in most environments, buried cables will encounter substantial water contact. Water reaching the fiber upon freezing and thawing can cause significant bending stresses that, in turn, induce fiber damage or unacceptable transmission losses.
A wide variety of encapsulants have been contemplated for encapsulated or filled cables. The use of liquid or gelled encapsulants such as an oil-extended styrene/ethylene-butene/styrene block copolymer has been suggested. (See, for example, The Proceedings of the Thirty-First Wire and Cable Symposium, U.S. Army Communications-Electronics Command (CECOM), Fort Monmouth, N.J., 1982, pp. 396-400.) In contrast, the use of substantially elastomeric materials to additionally limit microbending losses has also been suggested in U.S. Pat. No. 4,156,104 issued May 22, 1979 where it is indicated that HYTREL (HYTREL is a registered trademark of E. I. DuPont deNemours and Co., Inc.) polyester elastomers are employable. (Several HYTREL.RTM. polymers are available from E. I. duPont de Nemours & Company.) Although present cables show advantageous properties, it is certainly desirable to improve the transmission losses encountered through improvement to these structures.